1. Field of the Invention
The invention concerns photosensors and, in particular, those made in the form of a superimposition of amorphous silicon layers.
2. Description of the Prior Art
The most standard amorphous silicon photosensor is a PIN type diode formed by the superimposition of three semiconducting layers, one doped with a P type impurity, the next one non-doped (intrinsic or almost intrinsic), and the third one with N type doping.
For some years now, so-called NIPIN type, open-base phototransistors have also been proposed. These phototransistors are made by the superimposition of five layers of amorphous silicon. They have high sensitivity to light.
Such phototransistors have been described, for example, in the article "Amorphous Silicon Phototransistor on Glass Substrate" in the journal IEEE El. Dev. Lett. Vol EDL-8 No. 2, February 1987.
These phototransistors are well suited to the making of imaging matrices, capable of X and Y addressing, on large-area substrates.
But they have the drawback of having a remanent current after illumination, which is all the higher as their gain is greater. This means that a current is generated in the phototransistor during its illumination, and that it persists for a fairly long time after the illumination has stopped.
An aim of the invention is to propose a method of photoelectric detection enabling a reduction of this remanence.
FIG. 1 recalls the general structure of a NIPIN type phototransistor made of amorphous silicon. This structure is a stack of semiconductor layers made of amorphous silicon on a substrate 10 which is generally made of glass.
More precisely, the following are found, in the stated order, on the substrate:
a lower conductive layer 12 forming an emitter electrode of the phototransistor;
an intrinsic semiconducting thin layer 16;
a P type semiconducting thin layer 18, called a base layer;
a intrinsic semiconducting layer 20 which is far thicker than the first intrinsic layer 16;
an N type semiconducting thin layer 22 forming a collector region;
and a conducting layer 24, forming a collector electrode.
At least one of the electrodes, for example the collector electrode 24, is transparent or semi-transparent on the illumination source side of the phototransistor.
The base layer is not connected to an electrode. The phototransistor is an "overhead base" transistor.
With respect to the substrate, the order of the layers may be inverted, the collector being on the substrate side, and the other layers stacked on the collector (the collector region is the one located on that side where the thickest intrinsic layer is).
It is also possible to invert all the types of conductivity to make a PINIP transistor, the biases of the service voltages being then all inverted for the charge carriers are then holes (whereas they are electrons in the case of the NIPIN transistors).
FIGS. 2a to 2c represent the potential energy levels inside the semiconductor, showing the different potential barriers which get created and which get deformed in different conditions of operation. In these figures, the emitter is to the left, and the collector is to the right.
In FIG. 2a, the energy levels are represented in the case where a null potential difference Vec=0 is applied between the collector and the emitter of the transistor. The energy levels are identical in the emitter and the collector. There is a potential barrier in the middle: the energy increases in the least thick intrinsic region (16), gets stabilized in the P type base layer (18) and falls back in the thickest intrinsic region (20). The height of the potential barrier is designated by Vb0.
FIG. 2b represents the deformations of the energy levels when the phototransistor is biased with a view to being used as a photodetector.
The bias is a negative voltage Vec of the emitter with respect to the collector.
The potential barrier between the emitter and the base is lowered to a value Vb1 smaller than Vb0.
However, the lowering is not enough to obtain a notable injection of electrons of the emitter in the base.
But if the phototransistor, and notably the intrinsic zone on the collector side, is illuminated, electron-hole pairs are generated in this zone. The electrons are drawn to the collector side, and the holes to the base side, as a consequence of the electrical field prevailing in the intrinsic zone.
Holes collect in the base which is not connected to an external source. This accumulation is restricted only by the recombination of the holes and by the diffusion of the holes towards the emitter. This accumulation further lowers the emitter-base potential barrier to a value Vb2 for which the base-emitter junction is slightly forward biased. FIG. 2c represents this situation.
The transistor is then switched on, and a flux of electrons is injected from the emitter towards the base. This flux is greater than the flux of electrons generated by the light: it is the phototransistor effect.
When the illumination ceases, the holes stored in the base are not instantaneously removed. They are gradually discharged by getting "untrapped" and by diffusion towards the emitter or recombination. But, so long as their quantity remains sufficient, a current of electrons injected by the emitter persists and is collected by the collector.
This current constitutes a stray remanence effect. Experience shows that it is necessary to wait for several tens of milliseconds before it disappears to a proportion of 90 to 95%. Periods approaching even one second are needed for the dark current to recover its minimal value.
This phenomenon of remanence is far greater and, therefore, more troublesome than with a simple PIN photodiode in which the remanance is due only to the storage of carriers in the traps of the intrinsic zone. With the phototransistor, there is this storage in the traps of the intrinsic zone and, furthermore, there is the current that continues to be injected from the emitter to the base.
The solution in the case of the photodiodes, which can be transposed to the case of phototransistors, is to permanently illuminate the phototransistor with an ancillary source of continuous illumination that is superimposed on the illumination which it is desired to measure.
This constant illumination generates electron-hole pairs that fill the traps, and notably the traps with the deepest energy levels. These latter traps are, in effect, the longest to empty and it is these that increase the remanence effect.
When the useful illumination (signal illumination) ceases, the ancillary illumination is maintained, and the only holes that have to disappear are those that have been collected by the illumination of the signal. These holes disappear quickly. The others remain, but they do not need to be removed because the state of equilibrium, when there is no signal illumination, takes their presence into account. They exist both when there is illumination and when there is none, and it is enough to know this to determine the useful illumination signal by difference.
The drawback of this method is the existence of a permanent photocurrent due to the constant ancillary illumination. This current is a generator of noise. Furthermore, it is not necessarily identical from one phototransistor to another in one and the same matrix of phototransistors. Finally, it calls for the presence of an ancillary light source, which is not always easy to implement.